(1) Field of the Invention
The present invention relates to a lead-acid battery.
(2) Description of the Prior
The capacity of a lead-acid battery is, as known well, defined by the amounts of active materials in the negative and positive plates and sulfuric acid in the electrolyte.
In a lead-acid battery with a specific volume or a specific weight, in order to raise the active material utilization, it is necessary to decrease the active materials and increase the sulfuric acid volume, or to use sulfuric acid of high concentration. However, to raise the rate of utilization of the positive active material means to encourage softening and shedding of the active material, and also to accelerate grid corrosion. As a result, the cycle and the float life performance of the obtained battery are extremely shortened.
On the other hand, when the utilization of the negative active material is raised, the resistance of the active material to sulfation is lowered, and its life is shortened. When sulfuric acid of high concentration is used, the volume efficiency and weight efficiency of the battery can be raised. Nevertheless, since sulfuric acid of high concentration promotes the sulfation of the negative active material, it leads also to shortening of the battery life.
The plates used in the lead-acid battery may be roughly classified into the pasted type and the tubular type. Whichever plate might be used, it took an enormous amount of electric energy and time in the formation process to activate the unformed active material. Actually, the excess electric quantity in this formation process is necessary in order to activate the active material of the positive plate, in particular, and usually an electric quantity 2 to 4 times as much as required theoretically is consumed. This seems because the electron conductivity of lead dioxide which is the positive active material is low, and the electronic conductivity between active material particles is maintained only by the contact between particles. That is, when forming the positive plate, first the particles contacting with the current collector are oxidized to become lead dioxide. Next, the particles contacting the lead dioxide are formed, and this process is repeated to make up a formed plate. Therefore, particles far away from the current collector are formed finally in the final stage of the formation process. It means, within the formation process, that the density of the formation current in active material particles is high in the initial stage and low in the final stage, and on the whole the efficiency of the formation current is made much lower. For ease of formation, minium (Pb.sub.3 O.sub.4) may be added to the unformed active material. In this method, only the required electric quantity is decreased because the oxidation is advanced. It is therefore the same that the formation is started from the vicinity of the current collector so that the entire efficiency of the electric quantity for formation is lowered. Besides, minium is expensive, and its demerit of high cost is greater than the merit of saving of energy consumption for formation.
It is proposed in the U.S. Pat. No. 4,631,241 to mix graphite in the positive active material, hold the electrolyte between the layers, of the graphite and increase the pore volume in the positive plate so as to increase the capacity. Since graphite possesses an electron conductivity, it is possible to connect the particles of the active material electron conductively. However, the graphite disclosed in this cited reference is large in particle size, that is, 340 .mu.m, and the number is too few to connect among particles of the active material. The number of active material particles per unit volume is estimated around 1.times.10.sup.12 to 1.times.10.sup.16 in the case of positive plate, and 1.times.10.sup.9 to 1.times.10.sup.13 in the case of negative plate, from the specific surface area, pore size distribution and other data. By contrast, when graphite of this size of 340 .mu.m is added by 1 wt.% into the active material, the number of graphite particles per unit volume is about 500 to 1,000 at most, and this number is too small to expect an electron conductive connection among particles of the active material.
To improve bonding among active material particles, or between active material particles and grid, it is proposed to contain carbon fibers or electron conductive fibers in the active material in the Japanese Laid-Open Patents 61-128466, 54-0574, and 58-57264. Also in the Japanese Laid-Open Patent 49-103135, use of whiskers such as carbon fibers and lead is proposed. The proposed carbon fibers are 10 to 1,000 .mu.m in diameter as mentioned as "diameter 0.01 to 1.0 mm" in the Japanese Laid-Open Patent 54-105741, and since the diameter is large and surface area is small, according to the test by the present inventors, the number of contacts with the active material particles is small, and it is found impossible to open up the features of carbon to the maximum extent to enhance the conductivity dramatically.
In the Japanese Laid-Open Patent 49-103135, aside from such carbon fibers, use of "lead and other metal whisker" is shown. It is not mentioned, however, how to obtain this "lead or other metal whisker" and what characteristics, dimensions and form it possesses, and since an equivalent material cannot be obtained, its effect cannot be confirmed by the present inventors.
When the present inventors mixed lead fibers of 30 .mu.m in diameter cut to 2 mm size obtained by resonant vibration method into the active material, the density of the active material was raised and the contact density appeared to be enhanced, but, actually, the rate of utilization of active material and the charge acceptability were not improved.
According to the Japanese Laid-Open Patent 61-45565, "conductive synthetic resin fibers of 1 to 10 .mu.m in diameter" "obtained by mixing carbon powder or acid-proof metal powder into polyolefin or polyester synthetic resin" are mixed into the active material. However, the conductive synthetic resin fiber obtained in this manner is, if sufficient for reinforcement of the active material, not sufficient in the conductivity of the fibers themselves in order to enhance the electron conductivity among particles, and still more to mix fibers of 1 to 10 .mu.m will give rise to reduction of the apparent density of the active material, and therefore the functions of "enhancement of contact between the active material and the grid made of Pb-Ca alloy . . . and prevention of formation of barrier layer" as mentioned herein could not be confirmed.
Meanwhile, to improve the charge acceptability of the negative plate, carbon black is usually added. In this case, the carbon black mainly lowers the end of charging voltage. That is, by decreasing the hydrogen overvoltage of the negative plate, the current in the final stage of charging is increased. Carbon black has a very small particle size as compared with the graphite mentioned above. Therefore, addition of 0.2 wt.% will be a sufficient amount for contacting with individual particles of the active material. However, since the carbon black lacks length, it merely exists among the particles of the active material. In other words, tens or hundreds of active material particles cannot be mutually connected in parallel. The improvement of the charge acceptability of the negative plate by the addition of carbon black is to increase the charging current by lowering the charging voltage, and it does not mean that the current is made easier to flow in the individual particles of the active material. It means that a large current also flows in the positive plate in the final stage of charging, and the corrosion of the positive grid is accelerated by the increase of the overcharge amounts, which cannot be said, therefore, to be a preferable method for the life performance. That is, addition of carbon black to the negative plate does not lead to an essential improvement of charge acceptability.
It is hence a primary object of the invention to solve the problems of the prior art as discussed above by presenting a lead-acid battery which is
(1) enhanced in the rate of utilization of active materials of both positive plate and negative plate, possessing higher weight efficiency and volume efficiency than before, PA1 (2) improved in the charge acceptability (charge efficiency) of the active materials of both positive plate and negative plate, possessing a long cycle service life performance and a long float service life performance, PA1 (3) improved in the resistance to sulfation, capable of using the sulfuric acid of higher concentration, and greatly raised in the rate of utilization without sacrificing the life, PA1 (4) notably saved in the electric quantity required for formation of the positive plate, and PA1 (5) inexpensive.